Gas generant compositions using dicyanamide salts as fuel

Abstract: A gas generant composition includes a fuel, at least 25 wt% of which is an alkali, alkaline earth, and/or transition metal salt of dicyanamide and an oxidizer which is an ammonium, alkali metal and/or alkaline earth metal salt of a chlorate, perchlorate or nitrate.

Description: The present invention is directed to gas generant compositions suitable for automotive air bag restraint systems, and more particularly to gas generant systems using dicyanamide salts as fuel. Most automotive air bag restraint systems, presently in use, use gas generant compositions in which sodium azide is the principal fuel. Because of disadvantages with sodium aside, particularly instability in the presence of metallic impurities and toxicity, which presents a disposal problem for unfired gas generators, there is a desire to develop non-aside gas generant systems and a number of non-azide formulations have been proposed, e.g., US-A-4,369,079 and 5,015,309. However, to date, non-azide gas generants have not made significant commercial inroads. Materials that have been previously proposed for non-azide gas-generants include salts of bitetrazole, aminotetrazole, nitrotriazolone, triazolone, salts of nitrobarbituric acid, salts of nitroorotic acid, nitrouracil, salts of guanidine, and salts of amino-substituted guanidine, such as amino guanidine and triamino guanidine. Disadvantages of these materials include not being commercially available or not being available at a reasonable price and containing hydrogen in their chemical structure. It is advantageous to have fuels that contain little or preferably no hydrogen in their chemical structure. Upon combustion, fuels that contain hydrogen produce water vapor. Water vapor could be disadvantageous to bag performance at cold temperatures due to condensation. Heat capacity of the output gases is also increased with increased water content and potentially results in burns to the vehicle occupant upon inflation of the bag. US-A 4,386,979 teaches the use of cyanamide, dicyanodiamide (the dimerization product of cyanamide), and salts thereof as fuels in gas generant compositions. While some of the salts of cyanamide and dicyanodiamide are commercially available at a reasonable price and as salts of cyanamide contain no hydrogen, they have the disadvantage of not producing as great a quantity of gas upon combustion as would be desired. Further, they are not produced commercially in the purity that is required. The highest purity of commercial calcium cyanamide is 86 wt%, and the balance 14 wt% CaO renders the material unsuitable as a fuel. Dicyanodiamide has the disadvantage of a high hydrogen content. EP-A-519485 discloses a gas generant composition comprising 8.6 wt.% sodium dicyanamide and 20.5 wt.% of aminotetrazole as fuel components and 48.4 wt.% sodium nitrate and 22.5 wt.% zinc peroxide as oxidizers. The present invention comprises a gas generant composition comprising between 10 and 60 wt% of a fuel, at least 25 wt% up to 100% of which is selected from transition metal salts of dicyanamide and mixtures thereof, balance other fuel and between 40 and 90 wt% of an oxidizer selected from ammonium, alkali metal and alkaline earth metal chlorates, perchlorates, nitrates and mixtures thereof. Of transition metal dicyanamides, divalent transition metal dicyanamides are preferred, particularly cupric dicyanamide and zinc dicyanamide. The remainder of the fuel may be an azide or non-azide fuel, added to adjust burn temperature and gas output. Preferably, this other fuel is a non-azide fuel, such as those discussed above. Suitable cations may be lithium, potassium, sodium, magnesium, calcium, strontium, cerium and barium. In addition to these fuels containing no hydrogen, they are relatively non- toxic, and when formulated with an appropriate oxidizer, produce a non-toxic gas mixture upon ignition to inflate an automobile crash bag. Transition metal dicyanamides have certain advantages over alkali/alkaline earth dicyanamide compositions. For instance, cupric dicyanamide can be oxidized with an oxidizer such as a metal nitrate, e.g. strontium nitrate, to produce carbon dioxide, nitrogen and copper metal. When an alkali/alkaline earth dicyanamide, e.g. sodium dicyanamide, is combusted with an oxidizer such as strontium nitrate, the predicted products are carbon dioxide, nitrogen and a metal carbonate. The net result is higher gas yield from cupric dicyanamide, moles per 100 grams of generant. For instance, thermodynamic calculations performed by the Naval Weapons Center Propellant Evaluation Program (PEP) show that a stoichiometrically balanced mixture of strontium nitrate (68.1%) and sodium dicyanamide (31.9%) and strontium nitrate (36.6%) produce 1.61 moles of gas per 100 grams of generant. In addition to the higher gas yield, the resultant slag, copper metal, is easier to filter and more compatible than that produced by the sodium dicyanamide fuel. Similarly, zinc dicyanamide is better than sodium dicyanamide. Calculations show that a stoichiometrically balanced composition of zinc dicyanamide (34.14%) with strontium nitrate (65.85) produce 1.51 moles per 100 grams of generant which is higher than that produced by sodium dicyanamide and strontium nitrate. The oxidizer, which is used at a level of between 40 and 90 wt% is selected from ammonium, alkali metal and alkaline earth metal chlorates, perchlorates, nitrates and mixture thereof. Preferred oxidizers are nitrates. Optionally, a portion of the oxidizer may be a transition metal oxide, such as iron oxide or cupric oxide. In addition to their oxidizing function, these oxides provide hard particles, facilitating compaction of the composition into pellets or other consolidated solid shapes. For pellitization purposes, it is preferred that between 10 and 50 wt% of the total oxidizer content be a transition metal oxide, particularly cupric oxide. As is taught in US-A-5,139,588, the cations of the fuel salts and oxidizers are preferably mixtures of alkali metal cations, i.e., lithium, sodium and potassium, and alkaline earth metal cations, i.e., magnesium, calcium, strontium, barium and cerium. Upon combustion, the alkali cations form liquid slag components and the alkaline earth metal cations form solid slag components, the mixture of liquid and solid salts forming clinkers which can be readily removed from the gas stream by filtration. The ratio of solid to liquid combustion slag components may be adjusted by the ratio of alkaline earth metal cations to alkali metal cations. Alumina, silica or mixtures thereof may be added to scavenge corrosive alkali metal oxides, such as sodium oxide and potassium oxide. Accordingly, the composition of the present invention may contain alumina and/or silica at a level of between 0.5 and 30 wt%. The alumina and/or silica may be in the form of particulates or as fibers, such as fibers of various silica/alumina content. Alumina is generally preferred over silica, being a more efficient scavenger. A binder is optionally added at a level of up to 10%, preferably at least 0.5wt%. Suitable binder materials include but are not limited to molybdenum disulfide, graphite, polytetrafluroethylene, Viton® (a copolymer of vinylidene fluoride and hexafluoropropylene), nitrocellulose, polysaccharides, polyvinylpyrrolidones, polycarbonates, sodium silicate, calcium stearate, magnesium stearate and mixtures thereof. Preferred binder materials are molybdenum disulfide and polycarbonates. Alkali metal and alkaline earth metal carbonates and/or oxalates may optionally be added up to 10 wt%. These act as coolants, lowering the combustion temperature. Lower combustion temperatures minimize production of toxic gases, such as CO and NO x . Generally, if used, these coolants are used at a level of at least 1 wt%. As noted above, the alumina and/or silica may be in the form of fibers. Fibers help to mechanically reinforce the consolidated unburned material and subsequently consolidate slag material formed by burning the composition. Graphite fibers, e.g., up to 10 wt%, typically at least 1 wt%, may be also be used either alone as the sole fibrous material or in conjunction with other fibrous materials. The invention will now be described in greater detail by way of specific examples. Examples 1-4 (comparative) Gas generant compositions in accordance with the invention are formulated as follows, all amounts being in weight %: Example 1 2 3 4 Component Function Sodium Dicyanamide 31.9 28.66 23 19 Fuel Guanidine Nitrate 10 15 Co-Fuel Strontium Nitrate 68.1 61.34 57 51 Oxidizer Lithium Carbonate 5 10 15 Coolant Aluminum Oxide 5 Slag Former Thermochemical Calculations Tc (°K) 2444 2039 1977 1831 N 2 (mole/100g) 0.51 0.77 0.82 0.81 CO 2 (mole/100g) 0.49 0.53 0.47 0.44 H 2 O (mole/100g) 0 0 0.25 0.34 Example 5 (comparative) A generant composition in accordance with the invention are formulated as follows, all amounts being in weight %: Example 5 Component Function Sodium Dicyanamide 20.69 Fuel Guanidine Nitrate 11.76 Co-Fuel Strontium Nitrate 48.00 Oxidizer Lithium Carbonate 6.87 Coolant Cupric Oxide 12.75 Co-oxidizer/binder 100.00% Thermochemical Calculations Tc (°K) 1947 N 2 (mole/100g) 0.77 CO 2 (mole/100g) 0.45 H 2 O (mole/100g) 0.29 Examples 6 & 7 Examples of practical formulations of zinc and copper dicyanamide are shown in Table Ex. 6 and Ex.7 respectively. The compositions were prepared by mixing the materials in an aqueous slurry (approximately 25%), drying the composition, and screening the dried mixture. Burn rate slugs were pressed and burning rate measured at 6.89 MPa (1000 psi). Cupric Dicyanamide Formulations (Weight %) Mix 1 2 3 4 Component Cupric Dicyanamide 26.77 20.57 25.22 19.03 Guanidine nitrate 10 20 10 20 Lithium carbonate 10 10 10 10 Strontium nitrate 53.23 49.43 44.78 40.97 Cupric oxide 0 0 10 10 Thermochemical Calculations Rb (ips @ 6.89 MPa, 0.75 0.71 0.67 0.63 1000 psi) Moles/100 gm 1.70 1.95 1.60 1.86 Zinc Dicyanamide Formulations (Weight %) Mix 1 2 Component Zinc dicyanamide 34.14 24.46 Strontium Nitrate 65.86 60.54 Lithium carbonate 0 5 Ammonium diliturate 0 10 Thermochemical Calculations Rb (ips @ 6.89 MPa, 1000 psi) 0.65 0.7 Moles/100 gm. 1.51 1.60

Claims: A gas generant composition comprising between 10 and 60 wt% of a fuel, at least 25 wt% up to 100% of which is selected from transition metal salts of dicyanamide and mixtures thereof, balance other fuel and between 40 and 90 wt% of an oxidizer selected from ammonium, alkali metal and alkaline earth metal chlorates, perchlorates, nitrates and mixtures thereof. A gas generant composition according to claim 1, further containing between 0.5 and 10 wt% of a binder. A gas generant composition according to claim 2 wherein said binder is selected from molybdenum disulfide, graphite, polytetrafluoroethylene, vinyl fluoride/hexafluoropropylene copolymer, nitrocellulose, polysaccharides, polyvinylpyrrolidones, polycarbonates, sodium silicate, calcium stearate, magnesium stearate and mixtures thereof. A gas generant composition according to claim 2 wherein said binder comprises molybdenum disulfide or a polycarbonate. A gas generant composition according to any preceding claim further containing between 1 and 10 wt% of a coolant selected from alkali metal and alkaline earth metal carbonates, oxalates and mixtures thereof. A gas generant composition according to any preceding claim further containing between 1 and 10 wt% of graphite fibers. A gas generant composition according to any preceding claim further containing between 0.5 and 30 wt% alumina and/or silica. A gas generant composition according to any preceding claim containing, in addition to said salt(s) of dicyanamide, up to 50 wt% of a fuel selected from salts of bitetrazole, aminotetrazole, nitrotriazolone, triazolone, salts of nitrobarbituric acid, salts of nitroorotic acid, nitrouracil, salts of guanidine, salts of amino-substituted guanidine, and mixtures thereof. A gas generant composition according to any one of claims 1 to 8 wherein said salt of dicyanamide is cupric dicyanamide. A gas generant composition according to any one of claims 1 to 8 wherein said salt of dicyanamide is zinc dicyanamide. A gas generant composition according to any preceding claim wherein between 10 and 50 wt% of said oxidizer comprises a transition metal oxide or a mixture of transition metal oxides. A gas generant composition according to Claim 11 wherein said transition metal oxide is ferric oxide, cupric oxide or a mixture thereof. A gas generant composition according to claim 12 wherein said transition metal oxide is cupric oxide and said dicyanamide salt is cupric dicyanamide.

Inventors: BARNES MICHAEL W, DEPPERT THOMAS M, TAYLOR ROBERT D, BARNES, MICHAEL W., DEPPERT, THOMAS M., TAYLOR, ROBERT D.
Applicants: AUTOLIV ASP INC, AUTOLIV ASP, INC.
Application year: 1994
Main classification: C06D5
IPC codes: C06D5, C06B29, C06B43